In recent years, high integration of semiconductor devices has been advanced. In a case where a plurality of highly integrated semiconductor devices is arranged in a horizontal plane and is connected by wires for production, there is a concern that the wiring length is increased, the resistance of wires is increased, and the wiring delay is large.
Under the circumstances, there has been proposed the use of a three-dimensional integration technology that three-dimensionally stacks semiconductor devices. In this three-dimensional integration technology, two semiconductor wafers (hereinafter referred to as “wafers”) are bonded through the use of, e.g., a conventional bonding system. For example, the bonding system includes a surface modifying apparatus for modifying a surface of a wafer to be bonded, a surface hydrophilizing apparatus for hydrophilizing the surface of the wafer modified with the surface modifying apparatus, and a bonding apparatus for bonding the wafers whose surfaces are hydrophilized with the surface hydrophilizing apparatus. According to this bonding system, in the surface modifying apparatus, plasma treatment is performed on the surface of the wafer to modify the surface of the wafer. In the surface hydrophilizing apparatus, pure water is supplied to the surface of the wafer to hydrophilize the surface of the wafer. Thereafter, in the bonding apparatus, the wafers are bonded to each other by virtue of a van der Waals force and a hydrogen bond (intermolecular force).
The bonding apparatus includes an upper chuck configured to hold one wafer (hereinafter referred to as “upper wafer”) on the lower surface thereof, a lower chuck installed under the upper chuck and configured to hold another wafer (hereinafter referred to as “lower wafer”) on the upper surface thereof, and a pressing member provided in the upper chuck and configured to press the central portion of the upper wafer. In such a bonding apparatus, in a state in which the upper wafer held by the upper chuck and the lower wafer held by the lower chuck are disposed to face each other, the central portion of the upper wafer and the central portion of the lower wafer are pressed and brought into contact with each other by the pressing member, whereby the central portions are bonded to each other to form a bonding area. Thereafter, a so-called bonding wave occurs in which the bonding area expands from the central portion of the wafer to the outer peripheral portion thereof. Thus, the upper wafer and the lower wafer are bonded.
In order to suppress distortion of the laminated wafer after bonding, it is preferable that the bonding wave is uniformly (i.e., concentrically) expanded from the central portion of the wafer toward the outer peripheral portion thereof. However, in the bonding apparatus described above, the bonding wave is not monitored. Therefore, even if the bonding wave is unevenly expanded, it is not possible to grasp such an uneven expansion. Accordingly, there is room for improvement in the wafer bonding process of the related art.